In radio receivers, received radio frequency signals are frequently a combination of signals, some received directly from a transmitting antenna and some reflected from stationary and/or moving objects. In the worst case, the received signals from the direct and alternate path signals combine at the receiving antenna to cause destructive interference. Such interference makes decoding of the signals more difficult. Further, in some instances, interference can reduce the amplitude of the received signals to a level that is too low for reliable decoding by the receiver. Such amplitude reduction is sometimes referred to as multi-path fading.
One technique for improving signal reception under multi-path fading and weak signal conditions includes the use of multiple antennas and receiver circuits in an antenna diversity system. In a multi-chip antenna diversity system, multiple tuner circuits that are tuned to particular frequencies receive program content (channel information) from more than one direction or at slightly different positions. Such antenna diversity systems typically include processor circuitry configured to combine signals from the different tuners to produce an enhanced signal or to select a particular signal from a tuner having the strongest signal output.
Diversity reception makes use of statistically independent signal streams to reduce the impact of severe multipath-related channel fading. However, digital communications between the multiple tuner circuits and associated processing circuitry can radiate spectral energy at radio frequencies to which one or more of the tuner circuits are tuned, further complicating signal reception.
FIG. 1 illustrates a conventional FM antenna diversity system 100 having two or more antennas 1041-104N that are spaced apart at a known distance and are configured to receive a FM radio frequency signal. FM antenna diversity system 100 includes two or more FM tuner circuits 1021-102N that are each an individual chip that is connected to a respective one of independent antennas 1041-104N, and that is configured to receive FM radio frequency signals 105 from its given antenna 104 within a particular frequency band or channel to which the given FM tuner circuit 102 is tuned. The multiple FM tuner circuits 1021-102N are interconnected as shown by segments of uni-directional inter-chip (IC) communications link 120 via inter-chip communications circuitry provided for each tuner circuit 102 in the form of a respective IC communication link transmitter circuit 114 and a respective IC communication link receiver circuit 118 that are configured to communicate control/status information, intermediate frequency (IF), and audio data content from received FM radio frequency signals using inter-chip (IC) link frames in one direction from tuner circuit 102N toward tuner circuit 1021 across inter-chip communication link 120. These IC communication link (ICLINK) frames include frame synchronization information and three data channels, i.e., two synchronous streaming data channels that have a rate synchronous with ICLINK frame rate and a programmable word width of 24 or 32 bits, and one streaming/packetized messaging channel that sends a single data packet over multiple frames using any available bandwidth. The two synchronous streaming data channels are used for transmitting FM IF data or FM audio data. The single streaming/packetized messaging channel is used for sending messages between MCUs of the tuner circuits 102. Idle bits are sent when no data is available.
As shown in FIG. 1, system 100 also includes crystal oscillator 106 that provides an oscillator reference frequency that is used to produce a common reference clock signal 130 that is shared by the FM tuner circuits 1021-102N. Further information on antenna diversity systems and an IC communication link employing inter-chip link frames may be found in U.S. Pat. No. 8,548,031; U.S. Pat. No. 8,331,887; and United States Patent Application Publication No. 2011/0158298.
Still referring to FIG. 1, each of FM tuner circuits 1021-102N includes analog FM receiver circuitry 108 that is coupled to independently receive FM radio frequency (RF) signals 105 from its respective coupled antenna 104. Each analog FM receiver circuit 108 utilizes the reference clock signal 130 to mix with its respective received FM radio frequency signals 105 to produce IF FM signals, such as low IF or zero IF signals, that are provided to an analog to digital converter (ADC) 112 of the same respective tuner circuit 102. In the case of all tuner circuits except tuner circuit 102N, ADC 112 of each tuner circuit 102 digitizes the received IF signals to produce digitized versions of the IF signals that are provided as IF FM data to the digital signal processor (DSP) 116 of the same FM tuner circuit 102, which performs signal processing and/or demodulation operations to obtain an audio signal from the incoming FM signal. Further, as shown each tuner circuit 1021 to 102N includes a respective micro-controller unit (MCU) 110 that is configured to control operation of its respective tuner circuit 102, and that is coupled through a control interface to a data circuit (host processor) not shown.
In the conventional FM antenna diversity system 100 of FIG. 1, the DSP 116 of each FM tuner circuit 1021-102N-1 can demodulate the IF data provided from the ADC 112 of its own respective tuner circuit 102 to produce FM audio signals. The DSP 116 of each FM tuner circuit 1021-102N-1 can also combine the IF data of its own tuner circuit 102 with IF data received from a DSP 116 of another tuner circuit 102 across an inter-chip communications link 120 to produce combined FM IF data, which it can also demodulate to produce FM audio signals. In either case, each FM tuner circuit 1022-102N can send its demodulated FM audio signals out across inter-chip communications link 120 to the next preceding FM tuner circuit 102 as shown in FIG. 1. Since tuner circuit 102N of FIG. 1 has no DSP 116, ADC 112N of tuner circuit 102N provides its IF FM data across IC communication link 120 to DSP 112N-1 of tuner circuit 102N-1 where IF FM data of tuner circuit 102N is combined with IF FM data of tuner circuit 102N-1 and then demodulated and provided to the DSP 112N-2 of the next tuner circuit 102N-2 across IC communication link 120. In the conventional system 100 of FIG. 1, all final signal combining is performed by DSP 1161 of tuner circuit 1021 as shown to produce a combined demodulated FM audio signal.
Still referring to the conventional architecture of FIG. 1, the MCU 1101 of tuner circuit 1021 controls the DSP 1161 of tuner circuit 1021 to process the signal data received from other tuner circuits 1022-102N across IC communication link 120 according to a selected operating mode, i.e., a phase diversity mode, a switching antenna mode, or an alternate frequency scan mode.
In a phase diversity mode, the DSP 1161 synchronizes DSP frames including the digitized version of the IF signal and signal data within an IC communication link frame received across IC communication link 120 from upstream tuner circuit/s 1022-102N, performs maximal ratio combining or other similar digital signal processing techniques to coherently combine the IF signal from tuner circuit 1021 and the 1022-102N, and then demodulates this combined IF signal to provide the combined audio signal through a digital interface 150 to the data circuit.
In a switching antenna mode, each of the tuner circuits 1021-102N operate independently, and the signal reception is improved by continuously monitoring the signal quality metrics calculated from the digitized version of the IF signal as compared to the IF signal metrics received within IC communication link frames from the IC communication link 120. In this operating mode, the DSP 1161 is configured to select between the signals of the multiple antennas 1041-104N based on the signal metrics and to provide the stronger signal as a demodulated audio signal to the data circuit through the digital interface 150.
In an alternate frequency scan, the data circuit controls the multiple tuner circuits 1021-102N to use the IC communication link 120 to continue listening to a selected one of the multiple tuner circuits 1021-102N having the strongest signal and controls the other tuner circuits 120 to tune to the same content at another frequency to check the associated signal quality metrics. Depending on the results, the coupled data circuit may decide to control the multiple tuner circuits 1021-102N to operate in phase diversity mode or switched antenna diversity mode at the new frequency.